Voice quality can be measured by subjective or by objective methods. International groups carried extensive standardization work on this field. Subjective methods are listening experiments that involve a group of listeners that are presented with voice material. Each individual is asked to rate the speech quality according to a scale from 1 to 5. By averaging the opinion scores a number that reflects the speech quality is obtained. This number is called Mean Opinion Score (MOS) and it is well known for the quality characterization of speech coders. ITU Recommendation P.800 discusses subjective methods and provides with guidelines on how to obtain reliable and reproducible test results.
Objective measurement systems for speech quality measurement may use two signals as their input, namely an original signal (reference pattern) and the corresponding output signal after its transition through the network under test. The two signals are compared and an average score reflecting the voice quality is obtained. A popular objective method is ITU standard P.861 (1998) known as Perceptual Speech Quality Measurement (PSQM). PSQM was originally designed to objectively evaluate the quality of voice band (300-3400 HZ) speech codec, not to test live conditions over a communication channel.
Several factors affect voice quality in Voice over Packet networks: Delay, Jitter, Packet loss and Speech compression. The Pre-processing steps of Time alignment and Loudness adjustment are simple when the complete signals are available for storage and when the processing can be done off line. These tasks become very complicated if they need to be done in real time under network-degraded conditions. Voice quality measurements are extremely sensitive to any misadjustment during the Pre-processing steps. Misadjustments may be caused by erroneous detection of the beginning of the speech test material and also by missing parts of the speech test signal due to packet loss. They also include effects such as time scale modifications introduced by adaptive jitter buffers embedded in the Voice over Packet equipment. Such problems may severely degrade voice quality measurements.